to Downloads folder.
Then I installed it using GDebi Package Installer.
(Why did I behave this way? Because I couldn't find teamviewer via Synaptic Package Manager nor via Lubuntu Software Canter.)
Now I can see TeamViewer 12 in my pop-up menu, but if I click it nothing happens.
Did something go wrong? Did I make some mistake?
Many thanks

If you install openssh-server or samba then you can share the mount point of the CD and he can view the files over the network in his Windows file manager. Leaves your system free too as the file access happens in the background. Is this over the Internet or just the LAN?

Then port forward port 22 to your system and then your brother can connect to your WAN IP (use http://ipchicken.com to see this) on port 22, make him an account on the system and he can connect to the system via SFTP (secure) and view / download / upload files to your system.

You cannot use TeamViewer because your CPU doesn't have a feature it needs to run.